<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div>Header</div>

    <div id="app">app</div>

    <div>Footer</div>

    <!-- 加载 React。-->
    <!-- 注意: 部署时，将 "development.js" 替换为 "production.min.js"。-->
    <script src="https://unpkg.com/react@16/umd/react.development.js" crossorigin></script>
    <script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js" crossorigin></script>
    <script src="https://unpkg.com/babel-standalone@6/babel.min.js"></script>

    <script type="text/babel">
      let message = 'Hello World'

      function render() {
        ReactDOM.render(
          <div>
            <h2>{message}</h2>
            <button onClick={handleBtn}>改变文本</button>
          </div>,
          document.getElementById('app')
        )
      }

      function handleBtn() {
        message = 'Hello React'
        render()
        console.log(message)
      }

      render()
    </script>
  </body>
</html>
